1. Field of the Invention
The present invention relates to a power steering system, and more particularly, to a power steering system responsive to vehicle speed.
2. Description of the Related Art
The force required to turn a vehicle steering wheel is the greatest when the vehicle is stopped and decreases gradually therefrom as the vehicle runs faster. On the other hand, from the view point of steering stability of a vehicle, it is generally desirable that the force required to turn the steering wheel increases gradually as the vehicle runs at higher vehicle speed.
In view of these different requirements with respect to the performance of the steering system of a vehicle, it is known in the art of power steering to change the magnitude of the force exerted by the power steering means to the steering system for assisting the steering action by the driver according to vehicle speed so that said magnitude is decreased as the vehicle speed increases, as described in, for example, Japanese Patent Laid-open Publication 60-67264.
By such an arrangement of the power steering system, a light and comfortable steering action is available when a vehicle is driven at low speed wherein the stability performance of the steering system is of no substantial concern. During high speed driving of the vehicle, if the steering wheel turns so light that the steering system is liable to cause an oversteering action, the steering stability of the vehicle is lowered.
However, during the vehicle driving, it often happens that the vehicle runs along a long curved lane at high speed as on the expressway, with the steering wheel being held at a substantially turned condition. In this case, if the above-mentioned vehicle speed responsive power steering system is incorporated, because the vehicle is running at high speed, the magnitude of the force exerted by the power means for assisting the driver's steering action is low, and therefore, the driver must hold the steering wheel with his or her hands and arms generating a substantial force for a relatively long period.